Coated optical fibres are widely used in the communications field because of their ability to carry large amounts of information over long distances. In order to protect the fibres from physical damage during installation and also from subsequent deterioration due to environmental agencies, it is conventional to apply protective coatings to the freshly drawn fibres as an integral part of the production process.
Because of the difficulty of providing all the required physical properties in a single coating, optical fibres are frequently provided with two coatings, a soft primary (inner) buffer coating having an ability to compensate for the effects of differential thermal expansion and a secondary (outer) high modulus coating providing the necessary toughness and resistance to abrasion and chemical attack.
The normal method of installation involves pulling the fibres along previously laid cable ducts with the aid of ropes, and to avoid damage, it is necessary to overjacket the fibres with an expensive cable system.
In order to avoid these problems, it has been proposed in EP-A-0108590 to propel the fibre along a tubular pathway by fluid drag of a gaseous medium passed through the pathway in the desired direction of advance. In other words, the fibres, usually in sheathed multiple bundles, are blown into place on a cushion of air. Using this technique it is possible to "blow" optical fibres along microducts for long distances (several kilometers) without damage.
Fibres suitable for blowing require packaging which is cheaper and simpler than normal cable structure. A number of designs are known; in EPA 157 610 a unit is described in which fibres are surrounded by an inner sheath and in turn an outer layer of lower density to confer blowability.